1. Technical Field
The present invention relates generally to radio frequency (RF) devices. More particularly, the present invention relates to a multi-mode radio frequency front end module and associated switch circuitry.
2. Related Art
Wireless communications systems find application in numerous contexts involving data transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. Chief amongst these systems with respect to popularity and deployment is the mobile or cellular phone, and it has been estimated that there are over 4.6 billion subscriptions worldwide. Several different mobile phone technologies exist, including GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and UMTS (Universal Mobile Telecommunications System). Even within the same carrier, different mobile technologies or different generations of one family of mobile technologies may be deployed from one locality to another, as base station equipment may be upgraded at uneven intervals. Accordingly, mobile phone handsets typically have the capability of utilizing diverse mobile technologies, also referred to as multimode.
A fundamental component of mobile handsets, or any wireless communications system for that matter, is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver, with its digital baseband subsystem, encodes the digital data to a baseband signal and modules it with an RF carrier signal. The modulation utilized for GSM is Gaussian minimum shift keying, while EDGE utilizes an 8 phase shift keying format. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals. In almost all devices, whether single band or multiband, the transceiver is connected to a single antenna for size and cost reduction reasons.
Conventional mobile handset transceivers typically do not generate sufficient power or have sufficient sensitivity for reliable communications standing alone. Thus, additional conditioning of the RF signal is necessary. The circuitry between the transceiver and the antenna that provide this functionality is referred to as the front-end module, which include a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity. Various filter circuits such as band pass filters may also be included to provide a clean transmission signal at the antenna, and/or to protect the reception circuitry from external blocking signals reaching the antenna.
For multimode capabilities, particularly in relation to GSM-EDGE handsets, existing front end modules utilize a power amplifier specific to GSM for both GSM and EDGE. One power amplifier is fabricated on a single die for both GSM and EDGE due to size and cost constraints, even though EDGE has linear power requirements while GSM, by its nature, is non-linear. Therefore, substantial trade-offs are necessary to obtain both linear and non-linear performance from the single power amplifier. This mostly results in greater efficiency degradation in the EDGE mode, which may be below 25%. In comparison, efficiency in the GSM mode may be above 50%. Both of these efficiency measures are at a maximum rated output power, which is 30-33 dBm for GSM signals, and 26-27 dBm for EDGE signals. In actual operation, there may be further reductions in efficiency, in both EDGE and GSM modes, due to the power control modalities used, and the mobile nature of the handset, as different distances to base stations require different power outputs.
In addition to GSM and EDGE, conventional mobile handset transceivers also include W-CDMA or CDMA2k capabilities, which will be generally referred to as CDMA. These configurations include a separate power amplifier dedicated for CDMA. These multimode front end modules typically consist of two GSM-EDGE power amplifiers, two CDMA power amplifiers, and a single pole, seven terminal antenna switch module. Optionally, the CDMA power amplifier may include a DC-DC converter for increased efficiency.
As is understood, GSM-EDGE involves time-domain duplex (TDD), so transmit and receive channels utilize the same frequency while receiving and transmitting occur at different time intervals. Additionally, GSM-EDGE operates at two different frequency bands: low band, which by industry standards referred to as 850 MHz and 900 MHz, and a high band referred to as 1.8 GHz and 1.9 GHz. Accordingly, one of the GSM-EDGE power amplifiers is configured for the low bands of both GSM and EDGE, and the other of the GSM-EDGE power amplifiers is configured for the high bands of both GSM and EDGE. The GSM- and the EDGE-modulated RF signals are applied to a single pair of input lines of the integrated circuit.
GSM utilizes a constant envelope modulation scheme, and so the signal can pass through non-linear circuits without degrading the modulation information. Transmit power control is used to set an appropriate power level at the antenna for regulatory and battery conservation reasons. In most cases, for a GSM signal, a fixed RF power level is applied to the power amplifier, while power control is achieved by either adjusting the bias current or bias voltage. At medium and low power levels in GSM mode, battery consumption is reduced when the handset is in close proximity to a base station.
EDGE, however, utilizes a non-constant envelope modulation scheme, and the modulation information is highly degraded if the signal passes through non-linear circuit. Thus, as noted above, a linear power amplifier is necessary. Considering that GSM and EDGE modes share the same power amplifiers, back-off from saturated power helps achieve linear performance. To generate the appropriate power level, the input signal level to the power amplifier may be adjusted, or the bias current or the bias voltage can be adjusted.
Conventional multimode front end modules combine the two power amplifier outputs (high band/low band for a given mode) into a single antenna via a multi-port antenna switch module such as the single pole, seven position switch briefly mentioned above. GSM and EDGE received signals are passed through this switch and directed to the transceiver for further processing. The number of poles for the antenna switch is dependent on actual number of operating modes and frequencies and may differ from the seven mentioned above.
Besides the aforementioned configuration, more sophisticated architectures such as large signal polar loop may be employed, which utilizes highly efficient non-linear saturated power amplifiers for both GSM and EDGE signals. Despite improved battery consumption, circuit complexity of the transceiver and the front end module with such power amplifiers results in increased costs, and may find limited application only in high-end handsets.
CDMA also utilizes a non-constant envelope modulation scheme, so the standalone amplifiers typically employed in front end modules are linear. CDMA power amplifier efficiency is approximately 40% at maximum rated power, though it is possible to reach 50%. In order to maintain linear performance, RF isolators are typically connected to the power amplifier outputs based upon the assumption that there is a high antenna mismatch. An alternative solution is also often used without RF isolators while larger back-off from saturated power in this case results in lower efficiency. Unlike GSM and EDGE, CDMA mostly employs frequency domain duplex (FDD), meaning that transmission and reception operations are occurring simultaneously at different frequencies. The frequency difference can range between tens to several hundred MHz, so diplexers may be used. Furthermore, duplexers may be connected between the antenna and the power amplifier/low noise amplifier, and separate transmit and receive band pass filters may be employed.
The output power at the antenna may be adjusted by varying the power level of the RF signal to the CDMA power amplifier. Because the CDMA signal is continuous in the time domain, there is a large average current draw from the battery. In order to reduce power consumption, varying control modalities have been conceived, including adjustment of bias voltage and/or current, changing the power amplifier output matching network, and activating/deactivating certain power amplifier stages.
Due to these inherent operational incompatibilities, GSM/EDGE front end modules are fabricated as one module, and the CDMA front end modules are fabricated as a different module. The GSM/EDGE front end modules may be fabricated on a single chip, sometimes in conjunction with the antenna switch. As noted above, there are significant deficiencies with respect to the power/efficiency characteristics of such conventional front end modules. For efficient power control in either GSM-EDGE mode or CDMA mode, conventional devices utilize complicated transmit detector circuits in each transmit chain, as well as additional low pass or harmonics rejection filters. Accordingly, there is a need in the art for an improved multi-mode radio frequency front end module.